Suitability of Epitaxial Ga-As for X-Ray Imaging

The rate of indirect photon-electron conversion for scintillator materials coupled with arrays of photodiodes is at least 25 times smaller than the rate of direct conversion. We examine the conditions to be fulfilled by semiconductors undergoing such direct conversion to be applied to X-ray imaging. Bulk grown materials are not well-suited to this application because large defect concentrations give rise to strongly non-uniform electronic properties. We argue that epitaxial layers are suitable for use as imaging devices and we illustrate our argument using the case of thin epitaxial GaAs layers. Detectors made with such layers exhibit a good energy resolution, and a charge collection efficiency which approaches one.